Problem: Solve for $q$ : $q + 21 = 29$
Explanation: Subtract $21$ from both sides: $ \begin{eqnarray} \\ q + 21 &=& 29 \\ \\ {-21} && {-21} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{21 + q} &\hphantom{=}& \hphantom{29} \\ q &=& 29 {- 21} \\ \end{eqnarray} $ Simplify. $ q = 8$